The present invention relates to heat-exchange systems and, in particular, to systems for condensing the working gas of ocean thermal exchange power plants.
Although due to its almost unlimited availability and relative cleanliness, the use of ocean thermal energy as a power source is unusually attractive, there are a number of serious problems and difficulties which must be resolved if practical, cost-effective systems are to be widely used. In these systems which are known by the acronym OTEC, a working gas, such as the ammonia or propane, initially is heated by surface water temperatures to form a gas for driving turbines that produce electrical power for transmission to shore stations. The gas then is condensed for return to the evaporator, the condensing being achieved by using the cold temperature of relatively deep ocean water.
One of the more difficult problems involves the huge amounts of cold sea water which must be provided to the condenser heat-exchanger and the fact that these amounts either must be pumped to the surface from ocean depths or the heat-exchanger itself must be placed deep in the ocean. The requirement for large amounts of condensing sea water exists because of the small temperature differential and consequently low thermal efficiency. The system requires extremely large heat transfer surfaces which necessitate heat exchangers approximately the size of a seven story building. The pump requirements, piping size, drag forces, etc. must be commensurably large. The alternate system in which the large heat-exchanger is placed at the cold water depth presents other problems such as the need to move the working gas from the surface to the depth and to return the condensed fluid back to the evaporator at the surface. Another difficult problem is that the large heat-exchange structures must be firmly secured in the ocean depth and this need, aside from the resulting expense of the structure, seriously complicates maintenance and repairs.
Operation of OTEC systems in the ocean water involves a further complication in that the tubes of the heat-exchanger are susceptible to bio-fouling which results from bio-activity promoted by the nutrients present in the cold, sub-surface water. Bio-fouling of heat-exchanger tubes materially reduces or degrades their effectiveness so that steps must be taken either to avoid or to remove the bio-fouling during operation. In fact, this fouling is one of the largest problems facing the industry. Also, it is one that is endemic in that the nutrient gradient exists at the ocean depths where the essential cold water is found.
It is therefore an object of the present invention to provide an ocean thermal exchange system in which the size and the cost of the pumping, piping and the heat-exchange structure are effectively reduced.
Another object is to reduce the amounts of cold sea water transported to the surface, thus reducing size, costs and power requirements.
A further object is to increase the efficiency of these systems by applying material phase transformation phenomena to the heat-exchange operations.
Still another object is to provide a system which, in effect, is self cleaning insofar as bio-fouling is involved.
Other objects will become more apparent in the ensuing description.
Generally, the objects are achieved by employing a slurry of particles which undergo a phase transformation, such as a solid-liquid transformation, at a temperature between that of the cold, subsurface water and the condenser reject temperature. The slurry is formed by mixing the particles with cold sea water at the depth of the cold water and by delivering the slurry to the ocean surface where its coldness is used to condense the working gas of the energy conversion systems. Release of latent heat of fusion helps to maintain the cold water wter temperature. Use of the slurry minimizes the amounts of cold water pumped to the surface and it reduces size requirements, costs and parasitic pumping losses.